(Bug 5073) Use $background-color instead of $body-bg

$body-bg can contain images, etc, so we can't use darken with this. So
just use $background-color which only contains a color.

Also add this variable to Lynx, which doesn't currently have it (was
using the default before)
Merge pull request #637 from afuna/background-color-bug5073

(Bug 5073) Use $background-color instead of $body-bg